JSON-Serialisation leads to problem on Zeebe upgrade

[stephanpelikan]

1. JSON-serialization is used to pack events into Kafka message bodies.
2. In the case Zeebe introduces new Enum-values the clients do not know the new values on deserialization.
3. Updating the client first might not be an option:
   1. too many clients
   2. upgrading the client might not be backward-compatible (gRPC) to previous Zeebe version

Possible solutions:

1. Use schema based serialisation (Protobuf, Avro, etc.)
   1. ignore unknown events (introduced by new Zeebe version)
   2. every enum also includes an "unknown" value which will be used for new enum-values not known by the schema
   3. upgrade serialization eagerly, deserialization lazy
2. Other suggestions are welcome